The demand for new and improved electronic devices, such as cellular phones, notebook computers and compact camcorders, has demanded energy storage devices having increasingly higher specific energy densities. For example, the telecommunication industry is actively seeking alternate energy storage devices for its outside plant back-up power sources for telecommunication stations to replace the current standard valve-regulated lead acid batteries. Moreover, the automotive industry is in need of high specific energy density batteries for the developing electric and hybrid vehicles market. A number of advanced battery technologies have recently been developed to service these devices and markets, such as metal hydride (e.g., Ni-MH), nickel-cadmium (Ni—Cd), lithium batteries with liquid electrolytes and recently, lithium batteries with polymer electrolytes.
Lithium batteries have been introduced into the market because of their high energy densities. Lithium is atomic number three on the periodic table of elements, having the lightest atomic weight and highest energy density of any solid material. As a result, lithium is a preferred material for batteries, having very high energy density. Lithium batteries are also desirable because they have a high unit cell voltage of up to approximately 4.2 V, as compared to approximately 1.5 V for both Ni—Cd and Ni-MH cells.
Lithium batteries can be either lithium ion batteries or lithium metal batteries. Lithium ion batteries intercalate lithium ions in a host material, such as graphite, to form the anode. On the other hand, lithium metal batteries use metallic lithium or lithium alloys for the anode.
The electrolyte used in lithium batteries can be a liquid or a polymer based electrolyte. Lithium batteries including liquid electrolytes have been on the market for several years. Lithium ion rechargeable batteries having liquid electrolytes are currently mass produced for applications such as notebook computers, camcorders and cellular telephones. However, lithium batteries having liquid electrolyte technology have several major drawbacks. These drawbacks relate to cost and safety and stem from use of a liquid electrolyte. The liquid electrolyte generally requires packaging in rigid hermetically sealed metal “cans” which can reduce energy density. In addition, for safety reasons, lithium ion rechargeable batteries and lithium-metal primary batteries having liquid electrolytes are designed to vent automatically when certain abuse conditions exist, such as a substantial increase in internal pressure which can be caused by internal or external overheating. If the cell is not vented under extreme pressure, it can explode because the liquid electrolyte used in liquid Li cells is extremely flammable.
Lithium batteries having solid polymer electrolytes represent an evolving alternative to lithium batteries having liquid electrolytes. Solid polymer electrodes are generally gel type electrolytes which trap solvent and salt in pores of the polymer to provide a medium for ionic conduction. Typical polymer electrolytes comprise polyethylene oxide (PEO), polyether based polymers and other polymers which are configured as gels, such as polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) and polyvinylidine fluoride (PVDF). The polymer electrolyte generally functions as a separator, being interposed between the cathode and anode films of the battery.
Because its electrolyte is generally a non-volatile material which does not generally under normal operating conditions leak, a lithium battery having a polymer electrolyte is intrinsically safer than a lithium battery having a liquid electrolyte. Moreover, polymer electrolytes eliminate the need for venting and package pressure control which are generally required for operation of lithium batteries having liquid electrolytes. Thus, polymer electrolytes make it possible to use a soft outer case such as a metal plastic laminate bag, resulting in improvement in weight and thickness, when compared to liquid electrolyte can-type Li batteries.
Each cathode, separator and anode combination forms a unit battery cell. Practical lithium batteries, such as those having polymer electrolytes, are generally prepared by stacking a number of battery cells in series and/or parallel to achieve desired battery capacity.
Lithium metal polymer (LMP) rechargeable batteries offer improved performance as compared to Li ion batteries, particularly higher capacity. LMP batteries result from the lamination/assembly of three types of main thin films: a film of positive electrode comprising a mixture of a polymer and an electrochemically active material such as lithium vanadium oxide, an electrolyte film separator made of a polymer and a lithium salt, and a negative electrode film comprising metallic lithium or a lithium alloy. A known limitation of LMP batteries, especially for automotive applications is that the polymers presently used provide their peak ionic conductivity at elevated temperatures. These LMP batteries must therefore be heated to produce peak power output.
A problem with Li metal and lithium alloy batteries is that lithium, in its metallic form, is highly reactive. As such, it presents unique difficulties in rechargeable configurations. Repeated charge/discharge cycles can cause a build-up of surface irregularities on the lithium metal electrode. External pressure is generally necessary for prolonged performance of a lithium metal battery.
These irregular structures, known as dendrites, can grow to such an extent that they penetrate the separator between positive and negative electrodes and create an internal short circuit. At best, this phenomenon shortens the useful life of a rechargeable Li-metal battery to generally about 900 cycles or less.
Many performance parameters of lithium batteries are associated with the electrolyte choice, and the interaction of the selected electrolyte with the cathode and anode materials used. High electrolyte ionic conductivity generally results in improved battery performance. The ionic conductivity of gel polymer electrolytes have been reported to be as high as approximately 10−4 S/cm at 25° C. However, it is desirable for the ionic conductivity of the polymer electrolyte to reach even higher values for some battery applications. In addition, it would also be desirable to enhance the electrochemical stability of the polymer electrolyte towards anode and cathode materials to improve battery reliability, as well as storage and cycling characteristics.
While gel polymer electrolytes represent an improvement over liquid electrolytes in terms of safety and manufacturability, safety issues remain because gel polymers trap solvent on its pores and under extreme conditions (e.g. heat and/or pressure) can still escape and cause injury. In addition, gel polymer electrolytes cannot generally operate over a broad temperature range because the gel generally freezes at low temperatures and reacts with other battery components or melts at elevated temperatures. Moreover, electrode instability and resulting poor cycling characteristics, particularly for metallic lithium containing anodes, limits possible applications for such batteries formed with gel polymer electrolytes.
Alternative polymer materials have been actively investigated to provide improved characteristics over available polymer choices. For example, U.S. Pat. No. 5,888,672 to Gustafson et al. ('672 patent) discloses a polyimide electrolyte and a battery formed from the same which operates at room temperature and over a broad range of temperatures. The polyimides disclosed are soluble in several solvents and are substantially amorphous. When mixed with a lithium salt, the resulting polyimide based electrolytes provide surprisingly high ionic conductivity. The electrolytes disclosed in '672 are all optically opaque which evidences some phase separation of the various components comprising the electrolyte. Although the electrolytes disclosed by the '672 patent can be used to form a polymer electrolyte and a battery therefrom which provides an improved operating temperature range, ease of manufacture, and improved safety over batteries formed from conventional gel polymer electrolytes, it would be helpful if the electrolyte stability over temperature and pressure as well as the ionic conductivity could be improved.